The well-known ball grid array package (BGA) is to place solder balls on a substrate in an array for mounting on a printed circuit board (PCB) by surface mount technology, SMT, to achieve the maximum pin counts within the minimum footprint.
As shown in FIG. 1, a conventional BGA package 100 includes a substrate 110, a chip 120 and a plurality of solder balls 140. The chip 120 is disposed on the top surface 111 of the substrate 110 wherein the chip 120 is a bumped chip with a plurality of bumps 122 disposed on the active surface 121 of the chip 120. Then the chip 120 is flipped and electrically connected to the substrate 110 by means of the bumps 122. A plurality of solder balls 140 arranging in an array are disposed on the bottom surface 112 of the substrate 110 to electrically connect to the SMT surface 11 of the PCB 10. Normally, an underfill 130 is disposed between the chip 120 and the substrate 110 to encapsulate the bumps 122.
When the BGA package 100 is attached to the PCB 10 by SMT, in order to electrically connect the solder balls 140 to the connecting pads of the PCB, a high-temperature reflow process is necessary. However, during the reflow process, the substrate 110 can be easily warpaged due to the high reflowing temperature. Normally, the maximum stress is concentrated on parts of the solder balls 140 close to the edges of the substrate 110. The solder balls 140 can be easily broken leading to electrically open. In order to increase the bonding strength of the solder balls 140, larger solder balls are preferred but will easily cause bridging during SMT processes.